A novel anti-obesity agent which inhibits fatty acid synthesis and stimulates fatty acid oxidation is described. The hydrochloride salt of Ro 22-0654 (4-amino-5-ethyl-3-thiophenecarboxylic acid methyl ester) is a potent inhibitor of fatty acid and cholesterol synthesis in rat-isolated hepatocytes. Hepatic fatty acid synthesis was inhibited in vivo in a dose-dependent fashion with a duration of action of approximately 8 h. Adipose tissue fatty acid synthesis was also inhibited in vivo. Inhibition of fatty acid synthesis occurs without any apparent effect on several lipogenic enzymes, the tricarboxylic acid cycle, and the pentose phosphate shunt. Ro 22-0654 also stimulated fatty acid oxidation (in vitro) and lipolysis (in vivo). In long-term studies (2 months), Ro 22-0654 decreased body weight gain in Sprague Dawley and genetically-obese Zucker rats. Food intake was decreased following a single dose and for several days during chronic treatment. However, while food intake quickly returned to normal, body weight gain remained lower in treated rats. The effect on body weight gain can be ascribed to decreased total body lipid content in the absence of an effect on lean body mass. It is suggested that Ro 22-0654 may have utility in the treatment of human obesity.